The immature B cell represents an important window in B lymphocyte development, for it is at this stage that cells expressing self-reactive specificities are identified and eliminated. In isolation, immature B cells undergo apoptosis in response to BCR engagement while mature B cells initiate activation programs and are resistant to apoptosis. These data suggest that the differential fate responses of the immature and mature stage B cells following BCR engagement are due to intrinsic signaling events. Our previous studies demonstrated that while BCR engagement of mature B cells results in an elevation in intracytoplasmic Ca2+ levels (Ca2+) i that is accompanied by a concomitant activation of protein kinase C (PKC), immature B cells respond to BCR engagement by elevating (Ca2+) i in the relative absence of PKC activation. We hypothesize that, in the absence of PKC activation, Ca2+ -dependent signaling events play a causative role in the BCR-induced apoptosis of immature B cells. Our preliminary results indicate that the BCR-induced apoptotic response of immature B cells is relatively independent of an increase in (Ca2+)i and instead suggest that a depletion of endoplasmic reticulum (ER) Ca + stores may play a determinant role in this response. Depletion of ER Ca2+ stores results in a block in protein transport that is accompanied by a cellular stress response known as the unfolded protein response (UPR). The UPR comprises a unique "ER to nucleus" signaling pathway designed to sense the level of misfolded protein in the ER and remodel the secretory pathway to reduce it. Importantly, the UPR has a built in "fail-safe" mechanism; if the accumulation of misfolded protein is not reduced, an apoptotic response ensues. The central hypothesis of the studies proposed in this application is that the level and/or duration of ER stress and the associated UPR determines the fate of the B cell to BCR signaling. To specifically test this hypothesis, we propose to: 1) assess alterations in Ca2+ homeostasis following BCR crosslinking in immature and mature stage B cells, 2) assess the potential role of the UPR in regulating BCR-induced fate decisions , 3) assess whether accumulation of misfolded protein affects B cell development through UPR-mediated events, and 4) determine the effects of PKC activation on BCR-induced alterations in Ca2+ homeostasis and UPR induction. Understanding the molecular basis for these developmentally regulated BCR-induced fate decisions will provide insight into how these processes may be subverted in autoimmune disease states.